What exactly does it mean if you tore your acl?

Please see link...: "acl is the medical abbreviation for the anterior cruciate ligament. The acl is one of four major ligaments of the knee and stops the lower leg (tibia) from moving forward in relation to the thigh (femur) while providing rotational stability of the knee joint. Two bundles of fiber combine to make the acl..." http://www.Orthonc.Com/sites/default/files/forms/acl-db-web-page-rev-punched-redo.Pdf.